Nucleotides play an important role in the signal transduction of a cell. For example, the nucleotide cAMP is involved in the signal transduction of G-protein-coupled receptors (GPCRs). GPCRs form the largest class of pharmaceutical drug targets. In order to test the activity of potential new medicines on GPCRs in drug discovery, assays are needed which monitor the function of the receptors in their cellular environment. For high throughput screening, very early in the drug discovery process, these functional assays need to be simple with a low number of steps, sensitive to detect minute effects of early compounds and they need to comprise a robust readout to be applied in an automated fashion.
Functional assays for GPCRs can be set-up by detection of second messenger molecules reflecting the activation state of the receptor. Such a second messenger molecule is cyclic adenosine monophosphate (cAMP) formed upon modulation of the adenylyl cyclase activity. An overview on commercially available, functional assay kits that determine the level of cellular cAMP based on fluorescence or chemiluminescence readout is given by W. Thomsen et al. (Current Opinion in Biotechnology 2005, 16, 655-665). These kits are time-consuming and complex in operation and number of steps.